1. Field of the Invention
The present invention is related to the field of data processing methods for oil well logging and sampling. More specifically, the present invention relates to methods for determining properties of hydrocarbon mixtures and crude oils including molecular composition, molecular size, molecular weight, and molecular carbon number using nuclear magnetic resonance (NMR) data.
2. Background Art
Oil well logging and sampling tools include nuclear magnetic resonance (NMR) instruments. NMR instruments can be used to determine properties of earth formations, such as the fractional volume of pore space, the fractional volume of mobile fluid filling the pore space, and the porosity of earth formations. In addition, NMR data may be used to assess the content of brine and hydrocarbons in the formation. General background of NMR well logging is described in U.S. Pat. No. 6,140,817, assigned to the assignee hereof.
The signals measured by nuclear magnetic resonance (NMR) logging tools typically arise from the selected nuclei present in the probed volume. Because hydrogen nuclei are the most abundant and easily detectable, most NMR logging tools are tuned to detect hydrogen resonance signals (from either water or hydrocarbons). These hydrogen nuclei have different dynamic properties (e.g., diffusion rate and tumbling/rotation rate) that are dependent on their environments, such as the chemical structure and size of the molecules in which they reside. The different dynamic properties of these nuclei manifest themselves in different nuclear spin relaxation times (i.e., spin-lattice relaxation time (T1) and spin-spin relaxation time (T2); spin-lattice relaxation is also referred to as longitudinal relaxation, and spin-spin relaxation as transverse relaxation). For example, molecules in viscous oils cannot diffuse or tumble as fast as those in light oils. As a result, they have relatively short relaxation times. These observations suggest that NMR data (e.g., relaxation times) can provide information on molecular properties of hydrocarbons in the earth formations.